I was thinking about this today and came to this conclusion:

When women hear about this nice guy phenomenon, hear about these men's pain, just like all people, they want to fix it. Whenever someone is in pain, it's just a natural urge for human beings to want to fix the pain and make things better. Unfortunately the one way that the woman can think of to fix the pain (if she knows the guy personally or could conceivably do so) is to become the nice guy's girlfriend. She doesn't want to do that, clearly, and she feels guilty about not being able to do the one thing that would really, maybe, make things better (I know, maybe the nice guy wouldn't even have her, but if he is really desperate it's a fair bet that he would).

Since the only thing that would fix the pain is something she is unwilling to do, she has to get rid of the guilt somehow - she does this by devaluing his pain or dehumanising his person. If she can rewrite in her mind his deep loneliness as "a male sense of entitlement to sex in exchange for niceness tokens" - she can feel safe that she isn't the bad guy in all this. If she can think of him not as a fellow human suffering in this veil of tears, but instead as a "loser" or a "creep" then more the better, she relieves herself of the guilt of being unwilling to do the one thing she can do to help.

That's not to say she should just go be his girlfriend - pity relationships don't tend to work out that well, and I am pretty sure the guy in question would be horrified at the honest proposition of a pity relationship (although I am sure there's some people who'd settle for one out of sheer thirst). What the girl fails to realise is that, for all the discomfort she feels at not being able to help - she shouldn't then add insult to injury by devaluing the guy or his pain. It's better to do nothing than to make things worse.

It's kind of like when I see a beggar and I don't want to give them money my first thought is "oh well they're probably scammers anyway". This absolves me of my guilt for lack of charity. 9/10 times they might be scammers - but it's not their identity as a scammer that made me cling tight to my money but my unwillingness to part with it.